Leakage of blood across the mitral valve occurs relatively commonly, and when moderate can contribute to poor athletic performance, congestive heart failure or sudden cardiac death. The mitral valve separates the left atrium from the left ventricle. When the heart is resting (diastole) the valve is open allowing freshly oxygenated blood from the lungs to flow freely from the atrium into the ventricle. The flow is facilitated by an atrial kick at the end of diastole caused by atrial contraction. This event can be heard as the 4th heart sound and it coincides with the P-wave on the electrocardiogram (ECG). This ‘kick’, which represents about 20% of ventricular fill is missing in horses with atrial fibrillation, explaining why these horses suffer from poor athletic performance at maximal exertion. The valve should completely close off the opening between the atrium and the ventricle during ventricular contraction (systole). This prevents blood returning to the atrium, and facilitates blood moving across the aortic valve into the circulation.
Anatomy
The mitral or left atrioventricular valve is commonly referred to as a bicuspid valve, although this is not strictly accurate. The valve does have two major leaflets or cusps, these are referred to as the anterior (septal, medial, or aortic) and posterior (parietal, lateral, marginal, or mural) cusps. When the valve is open during diastole the anterior (septal) cusp lies against the interventricular septum, and the posterior (parietal) leaflet lies against the left ventricular free wall. Between the two major cusps are the small left and right accessory or commissural cusps. These small cusps contain 1-3 ‘sub-leaflets’ each, the number varying between horses.
The major cusps are attached to the mitral annulus and supported by chordae tendineae that are anchored to the left and right papillary muscles. Chordae from the left (craniolateral or sub-auricular) papillary muscle supports the left halves of the two large cusps and the left commissural leaflet. Chordae from the right (medial; sub-atrial) papillary muscle supports the right halves of the two major cusps and the right commissural leaflet. The edge of each large cusp is supported continuous chorda that link both papillary muscles.
Causes
A small amount of leakage during heart contraction (systole) is not uncommon and is not associated with any signs. This may happen if one or more of the chords (strings) that prevent the valve from prolapsing into the atrium become stretched, causing the valve to bulge (flail) into the atrium. This puts more pressure on the adjacent chords and could potentially cause rupture, which can be catastrophic.
The valve is held in place by attachments to fibrous tissue (left and right fibrous trigones), collectively known as the mitral annulus. If there is chamber dilation on either side of annulus, the ring can spread apart causing the valve to become incompetent. For example, the ventricle can dilate in response to leakage across the aortic valve and the atrium dilates in response to leakage across the mitral valve.
The valve cusps are susceptible to nodular thickening caused by bacterial infection. The is referred to valvular endocarditis. The thickening can distort the valve causing incompetence. Unfortunately sterilizing the valve with antibiotics will not always resolve the leakage as the cusps may further distort with fibrosis during healing.
As the horse ages the valve may naturally distort due to ‘myxomatous degeneration‘ of the leaflets. This is similar to what happens in aged small breed dogs.
Consequences of Mitral valve regurgitation
Many horses with small leaks tend to have very little to no progression over time. However, horses with moderate to severe regurgitation will likely experience clinical signs. As expected the leakage during systole will lead to increased pressure within the left atrium as blood normally entering from the pulmonary veins will be ‘supplemented’ with blood from the left ventricle. This increased left atrial pressure will ultimately causes the atrium to dilate. This increased left atrial size predisposes the horse to atrial fibrillation. The increased left atrial pressure will be translated through pulmonary circulation, causing pulmonary oedema and ultimately dilation of the pulmonary artery and signs of right heart failure. Pulmonary artery rupture is a possible cause of sudden death in these horses. Many horses are often missed until signs of right-sided congestive heart failure. This is partly because non-exercising horses can ‘hide’ pulmonary oedema, exhibiting no signs, or lethargy and a cough.
Diagnosis
The murmur is usually holosystolic (throughout systole with S1 and S2 both audible). It us usually band-shaped, meaning the intensity is typically even throughout systole. The murmur becomes clearly audible when you move your stethoscope caudally and ventrally over the left thorax, until you reach the 5th intercostal space. The murmur is loudest (relative to heart sounds) in the left 5th intercostal space. The murmur may radiate caudally and dorsally such that it may be heard in the the 6th-8th intercostal spaces.
The grade of the murmur intensity is high variable, typically horses are not investigated until they are a minimum grade of III/VI. Having said this the grade can be misleading as the regurgitant jet could be directed to the left atrial free wall against the thorax (louder) or towards the intra-atrial septum (softer).
The ideal method of diagnosis is through echocardiography. This technique uses Doppler to confirm the location of the murmur and to grade the magnitude of the regurgitant jet. Important measurements are made including the internal dimensions of the left atrium and the size of the pulmonary artery at its base.
The critical importance in assessing horses with cardiac murmurs is not only to assess the origin and severity, but also to assess suitability for riding. This is to ensure safety for horse and safety for rider.
Tags: Cardiology